This invention relates to capacitively weighted surface acoustic wave devices or transducers, and more particularly to means for improving the frequency response characteristics of such devices in order to minimize, if not eliminate, dips in the response characteristic within the frequency pass band to which prior devices have been subject.
Surface acoustic wave devices usually comprise a piezoelectric substrate upon which two or more series of interdigitated fingers or electrodes, called transducers, are deposited. Typical materials for the substrate are quartz and lithium niobate. The interdigitated fingers frequently are made of aluminum and are deposited and formed by well-known semiconductor techniques.
Surface acoustic wave devices, commonly called SAWS, are finding increasing uses as filters, delay lines and the like in the frequency ranges of 10 MegaHertz to 1 GigaHertz or higher.
Capacitive weighting of the individual fingers is one form of achieving the desired frequency response characteristics of a surface acoustic wave device or transducer, particularly when used as a filter. The U.S. Pat. to Ronald C. Rosenfeld, No. 3,904,996 entitled "Capacitive Weighted Acoustic Surface Wave Filter" teaches this form of device. The interdigitated electrodes or fingers extend from transducer pads on each side of the device and a capacitor formed by the same semiconductor techniques, as for example by vacuum deposition, photolithographic techniques and chemical etching is associated with each one of the interdigitated fingers.
To achieve the desired form of frequency pass band, which is usually to achieve maximum rejection of the frequencies beyond the desired band and a rapid skirt roll-off from the flat portion of the response pattern to the sides or skirts thereof, in a capacitively weighted transducer, it is required that the weighting capacitors of the electrodes towards the ends of the transducer be much smaller in value than the electrode capacitance, i.e., the capacitance of the electrode fingers. However, it is known that the electrodes of a surface acoustic wave device where the weighting capacitance is much smaller than the capacitance of the electrode fingers reflect more acoustic surface wave energy than the finger electrodes where the weighting capacitance is approximately equal to the capacitance of the electrode fingers. Reference is made to Ecom report 72-0326-5 October, 1974 entitled "Subminiature Broad Band Filters" pages 31-46 by Rosenfeld et al (U.S. Army Electronics Command). The reflection from the outer electrodes or fingers of the transducer causes a dip in the frequency pass band as pointed out in the Ecom report referred to. This dip due to the reflection from the capacitively weighted outer electrodes has been associated with quarter wavelength or split-connected electrodes. Reference is made to the Ecom report above and Publication DeVries, et al "Reflection of a Surface Wave From Three Types of ID Transducers", 1972 Ultrasonics Symposium Proceedings, pp. 353-358.